Can a 8 speed derailleur be used for 7 speed?

[Bert1220]

I plan on buy: http://www.disraeligears.co.uk/Site/Shimano_Deore_XT_derailleur_(M739_SGS).html

but I only have 7 gears so I'm wondering if the limit screws can be adjusted so it won't throw the chain off on the missing gear.

[xerxes]

It should be OK, there's only 1.5 mm difference in the overall width of 7 and 8 speed cassettes according to Sheldon: http://www.sheldonbrown.com/k7.html

[cyclerUK]

This will work with no problems.
The majority of Shimano rear derailleurs are compatible no matter the number of sprockets.
The only consideration is with the width of the chain and you will have no trouble with this XT working a seven speed.

[nfmisso]

(07-28-2010, 10:11 PM)Bert1220 Wrote:  I plan on buy: http://www.disraeligears.co.uk/Site/Shimano_Deore_XT_derailleur_(M739_SGS).html

but I only have 7 gears so I'm wondering if the limit screws can be adjusted so it won't throw the chain off on the missing gear.

As other mentioned, no problem with the derailleur. The shifters are another matter, because the sprocket to sprocket spacing is different. If you are using friction shifters, no problem, if you are using clicking (ratcheting) shifters, they need to be Shimano compatible 7 speed shifters.